Vapor pressure control process for a blended product stream



W. C. HART July25, 1967 VAPOR PRESSURE CONTROL PROCESS FOR A BLENDED PRODUCT STREAM Filed Sept. 9, 1963 F iFR'C: l

FEED

&

I III FEED A TTORNEYS United States Patent ce 3,332,856 VAPOR PRESSURE CONTROL PROCESS FOR A g BLENDED PRODUCT STREAM Walter C. Hart, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware Filed Sept. 9, 1963, Ser. No. 307,646 8 Claims. (Cl. 203-2) ABSTRACT OF THE DISCLOSURE Process and apparatus for continuously producing a product hydrocarbon stream 25 of predetermined constant vapor pressure by continuously blending a first hydrocarbon stream 23 of variable but lower vapor pressure with a second hydrocarbon stream 9 of controlled higher vapor pressure suflicient to maintain said product stream at said constant vapor pressure. Said second hydrocarbon stream is produced by a distillation column 2 having a feed inlet means 1, a bottom product outlet for supplying said stream 9, a variable temperature reboiler 4, and an overhead vapor condenser and reflux accumulator 6,8, said accumulator having a liquid level operated control 34 controlling at 36 the temperature of said reboiler, and having a reflux line 7 connected to said column and containing means 26, 28 to control the rate of flow of reflux. The vapor pressure of said second hydrocarbon stream is controlled by a first vapor pressure controller 22 measuring the vapor pressure of the product stream relative to a setting of its desired vapor pressure and sending a first control signal to a second vapor pressure controller 24 which measures the vapor pressure of said second hydrocarbon stream relative to said first control signal and sends a second control signal to reset the reflux rate of said reflux control means, thereby adjusting the vapor pressure of said second hydrocarbon stream so as to maintain said product stream at a predetermined constant vapor pressure.

-This invention relates to separation of fluid streams. In another aspect this invention relates to the production of fluidstreams having predetermined volatilities. In another aspect this invention relates to the control of separation columns. Much of the commercial demand for liquefied hydrocarbons, such as one containing mostly propane, requires that-the hydrocarbons have a certain volatility. The volatilityof a hydrocarbon stream varies depending upon the quantity of light and heavy fractions therein. Most generally, hydrocarbons are separated by fractionation, 'frac tional distillation, distillation, and selective absorption. These separation processes do not produce a hydrocarbon fraction having a constant volatility because variations in the feed compositions change the quantities of the light and heavier components remaining with the desired hydrocarbon' fraction. Rigid control of such separation processes may produce a pure desired hydrocarbon frac 'tion; however, such stringent controls require loss of the desired fraction to be removed with the other fractions thereby reducing the quantity of production of the desired fraction and making such operation uneconomical. In the past, a hydrocarbon product having a predetermined 'vapor pressure has been obtained by blending low vapor 'pressure hydrocarbons and high vapor pressure hydrocarbons. This blendingis accomplished by storing hydrocarbon streams in 'a surge tank and then blending other hydrocarbon streams thereto. This method is not desirable because it requires storage space, does not allow variation in the volatility of the hydrocarbons obtained, and is not operationally and economically eflicient.

3,332,856 Patented July 25, 1967 Accordingly, it is an object of this invention to produce a fluid stream having a predetermined volatility. It is another object of this invention to provide an improved method for blending fluid streams to obtain a fluid stream having a predetermined volatility. It is still another object of this invention to provide control of a separation column to obtain a product having a predetermined volatility. Still another object of this invention is to provide method and control for producing a hydrocarbon stream of varying volatility for blending with a hydrocarbon stream of varying volatility to produce a blend having a predetermined volatility.

In accordance with this invention a liquid product of a separation column is blended with a similar liquid product having a varying and lower volatility to produce a liquid product of predetermined volatility by controlling a process variable of the separation column in response to a volatility measurement of the blended stream to vary the volatility of the liquid produced to maintain a predetermined volatility of the blended stream.

Other objects and advantages of this invention will be apparent from the description and reference to the drawing, which shows a schematic flow diagram and apparatus suitable for practicing the invention.

Referring to the drawing, a mixed liquid feed, containing for example propane and lighter materials, is introduced into separation column 2 through conduit 1. Heat is added to column 2 by reboiler inlet conduit 4 to effect the separation of ethane and lighter materials therefrom. The light materials vaporized in column 2 are removed therefrom as overhead through conduit 3, passed through condenser 6 and into accumulator 8. Noncondensible overhead product is removed from accumulator 8 through conduit 5. Liquid is removed from accumulator 8 and returned to column 2, as reflux, through line 7 and pump 10. Liquid, containing mainly propane, is removed as bottoms product from column 2 through conduit 9.

A mixed liquid feed, containing, for example, butane and lighter, is introduced to column 12 through conduit 11. Heat is added to column 12 by reboiler inlet conduit 14 to effect the separation of propane and lighter materials therefrom. The light materials vaporized in column 12 are removed through conduit 13, passed through condenser 16 and into accumulator 18. Noncondensible gases are removed from accumulator 18 through conduit 15. Liquid is removed from accumulator 18 through conduit 17 and pump 20. A portion of the liquid from conduit 17 is returned to column 12, as reflux, through conduit 19. Liquid, containing mainly butane, is removed as bottoms product from column 12 through conduit 21. The remainder of liquid in conduit 17 is passed through conduit 23, combined with the liquid in conduit 9, and passed through conduit 25 as product.

Vapor pressure recorder controller 22 is operatively connected through a transmitter to conduit 25. Controller 22 compares the vapor pressure of the blended hydrocarbon stream with a set point value which is representative of a predetermined vapor pressure and transmits a corrective signal to vapor pressure recorder controller 24 operatively connected through a transmitter to conduit 9. Controller 24 compares the vapor pressure of the hydrocarbon stream in conduit 9with the correction signal from controller 22 and produces a control signal which is transmitted to flow recorder controller 26, operatively connected to conduit 7. Controller 26 compares the flow of reflux through conduit 7 with the corrective signal from controller 24 and produces a control signal which is trans mitted to valve 28 in conduit 7. The signal from controleler 26 manipulates valve 28 to regulate the flow or reflux the blended hydrocarbon streams will have the predetermined volatility.

Presure controller 30, operatively connected to conduit 3, produces a signal to manipulate valve 32 in conduit 5 for regulation of vapor removal therethrough. Liquid level controller 34, operatively connected to accumulator 8, produces a signal which manipulates valve 36 to regulate the flow of heating medium through reboiler inlet conduit 4. Liquid level controller 38, operatively connected to the lower portion of column 2, produces a signal 4 charge of 860,000 gallons per day at a temperature of 200 F. through conduit 11. A reflux of 644,000 gallons per day at a temperature of 111 F. is returned to column 12 through conduit 19. Overhead product of 167,000 gallons per day at 129 F. is passed through conduit 23 and combined with the product in conduit 9 to form a propane stream having the volatility of 200 p.s.i.g. in conduit 25. A kettle product of 681,000 gallons per day at 260 F. is removed from column 12 through conduit 21. The vapor pressure of the propane stream in conduit apparatus for determining vapor pressure of a liquid is that described in U.S. Patent 3,037,375, which is incorporated by reference to this disclosure. The pressure measurement obtained therefrom is passed through a Bourdon tube for conversion to a signal operative with recorder controllers. The vapor pressure control may also be eflected by analyzing the liquid streams for the concentration of light constituents therein and controlling the operation of the separation column in response thereto to maintain a predetermined concentration of light materials in the product stream which will give a desired vapor pressure of the blended product. Such component concentration analysis may be made by the well known chromatographic analyzers.

Example A propane stream having approximately a vapor pressure of 200 p.s.i.g. is produced in accordance with this invention by operating column 2 as a deethanizer wherein 228,000 gallons per day of hydrocarbon feed at a temperature of 76 F. is charged through conduit 1. Reflux at the rate of about 109,000 gallons per day at 49 F. is returned to the column through conduit 7. A kettle product of about 90,000 gallons per day at 173 F. is removed from the column through conduit 9. The top of the column operates at a temperature of about 70 F.

Column 12, operating as a depropanizer, has a feed Which manipulates valve 40 in conduit 9 for regulation of 23 is lower than that desired in conduit 25 whereas the the bottoms product withdrawal from column 2. vapor pressure of the propane stream in conduit 9 is Pressure controller 42, operatively connected to congreater than that desired in conduit 25. When the vapor duit 13, produces a signal which manipulates valve 44 pressure of the propane in conduit 25 is below that of in conduit 15 for regulation of the venting therethrough. 5 the predetermined value, controller 22, operating in a Flow recorder controller 46, operatively connected to concascade system with controller 24, signals controller 26 duit 19, produces a sign to manipulate valve 48 in conduit to change the flow through conduit 7 by decreasing it to 19 for regulation of flow of reflux to column 12 theredecrease the reflux rate in column 2 which raises the through. Liquid level controller 50, operatively connected liquid level in accumulator 8, causing liquid level control to the lower portion of column 12, produces a signal to 20 34 to decrease the flow of heating fluid through valve 36 manipulate valve 52 in conduit 21 for regulation of the and reboiler 4 and allow more light material to pass bottoms product Withdrawal from column 12. Temperathrough conduit 9. Conversely, when the vapor pressure ture controller 54, operatively connected to the lower of the propane stream in conduit is greater than the portion of column 12, produces a signal which manippredetermined value, the control signal operates to in- -ulates valve 56 to regulate the flow of heating medium 25 crease the rate of flow through conduit 7 to column 2 through reboiler inlet conduit 14. which lowers the liquid level in accumulator 8, causing Controllers 22 and 24 can be any combination of sensliquid level control 34 to increase the flow of heating ing elements and devices for determining the vapor presfluid through valve 36 and reboiler 4 and thereby de sure of the liquid and converting such to a signal; either crease the quantity of light material passing through electrical, pneumatic, hydraulic, or mechanical; repre- C nd 9- sentative of such pressure which will be operative with the The compositions of the various streams used in the conventional recorder controllers and flow recorder cona ov eXampl ar listed elow in ta le form in pound trollers employed in the control system. One suitable moles.

TABLE Column 2 Column 12 Component (l methane 10.1 10.1 1.1 1.2 Urethane 65.9 56.6 9.3 10.9 10.7 CaplO ane.... 185.2 21.7 163.5 578.8 511.2 67.6 lO4tso utane 4.0 .1 3.9 306.8 12.3 294.5 n0 normal butane 0.7 0.7 390.9 0.5 390.4 1C5 is0pentane. 9.0 1. 0 8.0

Total 205.9 88.5 177 4 1,797 5 535 7 1,250 5 1.3

From the above example it is apparent that this invention permits the separation columns to be operated at conditions to obtain the maximum amount of recovery of the desired hydrocarbon and still allow the recovered hydrocarbon stream to have a predetermined volatility.

This invention is applicable to all fluid streams where the volatility of same is to be maintained at a certain value. Most generally, such fluids will be hydrocarbons having from 2' to 4 carbon atoms per molecule.

Reasonable variations and modifications of this invention will be apparent to one skilled in'the art without departing from the spirit and scope of this invention.

That which is claimed is:

1. Process for producing a product stream of constant volatility comprising:

blending a stream of higher vapor pressure with a stream of lower vapor pressure;

sensing the vapor pressure of said blended stream;

varying the reflux rate of a fractionating column producing said higher vapor pressure stream in response to said sensed vapor pressure whereby the vapor pressure of said higher vapor pressure stream is varied inversely with said sensed vapor pressure and said sensed vapor pressure is maintained constant.

2. Process for producing a product stream of constant volatility comprising: 1

blending a stream of higher vapor press re with a stream of lower vapor pressure;

sensing the vapor pressure of said blended stream and generating a first signal representative of the difference between said sensed vapor pressure and a predetermined vapor pressure;

comparing said first signal with a second signal representative of the vapor pressure of said higher vapor pressure stream and generating a third signal representative of the difference therebet'ween;

varying the reflux rate of a fractionating column producing said higher vapor pressure stream in response to said third signal whereby the vapor pressure of said higher vapor pressure stream is increased responsive to a decrease in the vapor pressure of said blended stream and decreased responsive to an increase in the vapor pressure of said blended stream.

3. Process of claim 2 wherein said higher vapor pressure stream is produced from a distillation column below the level of reflux introduction.

4. Process of claim 3 wherein said lower vapor pressure stream is produced from a distillation colunm above the level of reflux introduction.

5. Process for producing a blended hydrocarbon stream of a predetermined vapor pressure comprising the steps of:

fractionating a first hydrocarbon feed stream to produce an overhead product of lower vapor pressure than said predetermined vapor pressure;

fractionating a second hydrocarbon feed stream in a column provided with a variable rate of reflux to produce a bottoms product having a vapor pressure higher than said predetermined vapor pressure, and measuring the vapor pressure thereof;

blending said overhead product and said bottoms product to form said blended hydrocarbon stream;

measuring the vapor pressure of said blended stream and generating a first control signal as a function of the differential between the vapor pressure of said blended stream and said predetermined vapor pressure;

generating a second control signal as a function of said first control signal and the measured vapor pressure of said bottoms product; and

controlling said nate of reflux responsive to said second control signal whereby the vapor pressure of said bottoms product is varied to maintain said predetermined vapor pressure in said blended stream.

6. Apparatus for producing a fluid of constant vapor pressure comprising:

:a distillation column having reflux means to return overhead product stream to said column as reflux;

a first conduit adapted to withdraw a first fluid as a product of said distillation column;

a blending means adapted to blend a second fluid with said first fluid;

means for measuring the vapor pressure of said blended fluids and genenating a signal responsive thereto; and

means for varying the rate of flow in said reflux means responsive to said signal.

7. Apparatus for continuously producing a blended hydrocarbon stream of predetermined vapor pressure which comprises in combination:

a distillation column having feed inlet means continuously introducing a hydrocarbon feed;

a variable temperature reboiler associated with the lower portion of said column;

an accumulator;

a first conduit having a condenser therein for passing and condensing vapor from said column to said accumulator;

a second conduit having a valve therein for passing lfilquid from said accumulator to said column as rea first control means operated by the liquid level of condensate in said accumulator to increase the varia-ble temperature of said reboiler as said liquid level decreases and to decrease the variable temperature of said reboiler as said liquid level increases;

a third conduit for withdrawing liquid from said column;

:a fourth conduit communicating with said third conduit for continuously adding a fluid having a varying volatility lower than desired;

a first vapor pressure controller means operatively connected to said third conduit downstream of said fourth conduitdisposed to measure the vapor pressure therein relative to a set vapor pressure and to generate a first control signal which is a function of the difference between these pressures;

at second vapor pressure controller means operatively connected to said third conduit upstream of said fourth conduit disposed to measure the vapor pressure therein relative to said first signal and to generate a second control signal which is a function of the upstream vapor pressure and said first signal;

a first control communicating means disposed to communicate said first signal from said first to said second vapor pressure controller means;

a flow controller operatively connected to said valve in said second conduit to control flow of reflux liquid therethrough; and

a second control communicating means disposed to communicate said second signal from said second vapor pressure controller means to control said flow controller.

8. Apparatus for producing a blended hydrocarbon stream of predetermined volatility which comprises:

a distillation column having feed inlet means;

a reboiler associated with the lower portion of said column;

an accumulator;

a first conduit having a condenser therein for passing and condensing vapor from said column to said accumulator;

a second conduit having a valve therein for passing liquid from said accumulator to said column as reflux;

a third conduit for withdrawing liquid from said column;

a fourth conduit communicating with said third conduit for adding a fluid having a varying volatility lower than desired;

a first vapor pressure controller means operatively connected to said third conduit downstream of said fourth conduit;

a second vapor pressure recorder controller means operatively connected to said third conduit upstream of said fourth conduit;

a first control communicating means connecting said first and second vapor pressure recorder controller means;

a flow recorder controller operatively connected to said second conduit and valve therein; and

a second control communicating means connecting said second vapor pressure recorder controller means and said flow recorder controller.

References Cited UNITED STATES PATENTS 2,868,701 1/1959 Berger 202- 2,881,235 4/1959 Van Pool 1'96132 2,964,992 12/ 1960 Hurdle 137-93 3,037,375 6/1962 Jacobs et al. 73-53 3,200,883 8/1965 Phillips 202160 3,223,107 12/1965' Kleiss 137--93 3,224,210 12/ 1965 Albritton 202-460 NORMAN YUDKOFF, Primary Examiner.

WILBUR L. BASCOMB, 111., Examiner. 

1. PROCESS FOR PRODUCING A PRODUCT STREAM OF CONSTANT VOLATILITY COMPRISING: BLENDING A STREAM OF HIGHER VAPOR PRESSURE WITH A STREAM OF LOWER VAPOR PRESSURE; SENSING THE VAPOR PRESSURE OF SAID BLENDED STREAM; VARYING THE REFLUX RATE OF A FRACTIONATING COLUMN PRODUCING SAID HIGHER VAPOR PRESSURE STREAM IN RESPONSE TO SAID SENSED VAPOR PRESSURE WHEREBY THE VAPOR PRESSURE OF SAID HIGHER VAPOR PRESSURE STREAM IS VARIED INVERSELY WITH SAID SENSED VAPOR PRESSURE AND SAID SENSED VAPOR PRESSURE IS MAINTAINED CONSTANT. 